1. Field of the Invention
The present invention relates to an electro-conductive oxide showing excellent electro-conductivity, an electrode using the electro-conductive oxide and a method for manufacturing the same. The electro-conductive oxides of the present invention show not only excellent electro-conductivity but also excellent transparency all over the visible region and therefore they are particularly useful as, for example, electrodes for displays and solar cells, which require light transmission.
2. Related Art
Transparent electro-conductive materials, which are transparent in the visible region and electro-conductive, are widely used as transparent electrodes of various panel form displays such as liquid crystal displays and EL displays and transparent electrodes of solar cells. They are also used as defogging heaters of chilled show cases, heat ray reflecting films for window glasses of buildings and automobiles, anti-static coatings or electromagnetic wave shields of transparent articles and the like.
As transparent electro-conductive materials, generally used are metal oxide semi-conductors. Various metal oxide materials are proposed and examples thereof include tin oxide (SnO.sub.2), indium oxide doped with tin (ITO), Cd.sub.2 SnO.sub.4 and CdIn.sub.2 O.sub.4.
Transparency of such transparent electro-conductive materials relates to the fundamental absorption edge wavelength. The term "fundamental absorption edge wavelength" means a wavelength at which light absorbance of the material due to electron transition from a valence band to a conduction band begins to appear. The fundamental absorption edge wavelength may be determined by the reflection method or the transmission method using a spectrophotometer. ITO has its absorption edge around 450 nm and does not absorb light of a wavelength longer than that wavelength. Therefore, it is transparent substantially all over the visible region except for the short wavelength region. On the other hand, it has a carrier concentration comparable to that of metals and a carrier mobility relatively large as an oxide and so it has a high electro-conductivity more than 1000 S/cm. Therefore, among the above-mentioned materials, ITO is particularly widely used.
Various panel form displays are widely used for various electric appliances including telephones, laundry machines, rice cookers, game machines, portable televisions, computers and wordprocessors. In particular, in note-book type personal computers, word-processors and the like, large panel form displays having a diagonal length of about 10 inches have become prevalent. In addition, researches of further larger panel form displays are continued for use in wall-televisions and the like.
Hitherto, ITO has been used also for transparent electrodes of panel form displays. However, as described hereinbefore, ITO has its fundamental absorption edge wavelength of 450 nm and hence shows poor transparency in the short wavelength region of the visible region (less than 450 nm). Therefore, since a larger thickness of ITO electrodes cause their coloration, a thinner thickness has been preferred. On the other hand, a larger thickness is preferred from the view point of reduction of electric resistance, i.e., reduction of power consumption. Therefore, a suitable thickness of transparent electrodes has been selected considering their transparency and electric resistance.
However, transparent electrodes for panel form displays of a larger size have a longer distance between the ends of the electrode surface and hence electric resistance between the ends is increased. In addition, displays of higher resolution must have a smaller line width of transparent electrodes and this also leads to higher resistance. On the other hand, if the electrode thickness is made larger to decrease electric resistance, it may cause practically unacceptable coloration. That is, conventional ITO practically used so far as materials for transparent electrodes as it is cannot provide large transparent electrodes having both satisfactory transparency and electro-conductivity.
For these reasons, it has been desired to develop materials showing transparency in a short wavelength region of the visible region, i.e., shorter than 450 nm, and high electro-conductivity.
For example, a spinel compound, ZnGa.sub.2 O.sub.4, was reported as a material having its absorption edge in the short wavelength region below 450 nm, and ZnGa.sub.2 O.sub.4 shows its absorption edge at 250 nm. However, it shows low electro-conductivity as low as 30 S/cm (The Ceramic Association of Japan, Preprints p585, (1993)).
A trirutile type compound, CdSb.sub.2 O.sub.6, has also been known. CdSb.sub.2 O.sub.6 shows its absorption edge at 350 nm. However, it also shows low electro-conductivity as low as 40 S/cm (The 54th Congress of the Society of Applied Physics, Preprints, vol. 2, p502).
That is, there has been known no materials having an absorption edge at a wavelength shorter than 450 nm and showing electro-conductivity comparable to or higher than that of ITO.
Therefore, the object of the present invention is to provide novel materials which do not cause coloration even with a thickness larger than that of conventional ITO films because they have an absorption edge at a wavelength shorter than 450 nm and show electro-conductivity comparable to or higher than that of ITO.
Another object of the present invention is to provide electrodes comprising the above-mentioned novel materials, which are useful for liquid crystal displays, EL displays, solar cells and the like.